1. Field of the Invention
The present invention relates to amplifier circuits. More particularly, the present invention is directed to a resistor network suited for operational amplifiers.
2. Description of the Related Art
The operational amplifier is a basic analog building block common to various electronic functions performed in instrumentation, computation and control circuits. A design of operational amplifier that facilitates sensing desired signals embedded in noise signals is known as a differential operational amplifier.
Referring to FIG. 1, a conventional operational amplifier 10 includes a first input In1, a second input In2, a first output, Out1, and a second output, Out2. The amplifier 10 facilitates discrimination between desired signals from noise signals by amplifying the differential signals applied to the first and second inputs In1, and In1. The noise signals that are common to both inputs, In1, and In2, i.e., common-mode signals are rejected. The effectiveness of the amplifier to reject common-mode signals is usually measured as a ratio of differential voltage gain to the common-mode voltage gain, referred to as the common-mode rejection ratio (CMRR).
Both the differential gain and CMRR are a function of the resistor networks 30a and 30b. Each of the resistor networks 30a and 30b include a feedback resistor 32a, 32b, respectively. An input resistor 34a and 34b is also included in each resistor network 30a and 30b, respectively. The differential gain, AD, is defined as follows:
AD=RFEEDBACK/RINPUTxe2x80x83xe2x80x83(1)
where RFEEDBACK corresponds to resistors 32a and 32b and RINPUT corresponds to resistors 34a and 34b. As discussed above, the CMRR is defined as follows:
CMRR=AD/Acmxe2x80x83xe2x80x83(2)
where ACM is the common-mode gain. The ACM is, however, approximated as follows:
ACMxe2x88x9d[(xcex94RINPUT/RINPUT)xe2x88x92(xcex94RFEEDBACK/RFEEDBACK)]xe2x80x83xe2x80x83(3)
where xcex94RINPUT=(value of resistor 34bxe2x88x92value of resistor 34a) and xcex94RFEEDBACK=(value of resistor 32bxe2x88x92value of resistor 32a). From the foregoing, it is seen that precise control of the CMRR, differential gain, AD, and common-mode gain ACM requires accurate control of the values of the RFEEDBACK and RINPUT resistors.
What is needed, therefore, is an amplifier that facilitates precise control of the resistors included in the resistor network associated therewith.
Provided is a circuit and method for operating the same that features an adjustable resistor network to enable varying the operational characteristics of an amplifier. To that end, the resistor network includes primary resistors connected in series with a plurality of adjustment resistors connected to the output of an operational amplifier. A switching network is connected between the resistor network and the input of the operational amplifier. The resistor network includes a feedback resistor and an input resistor, with the feedback resistor being coupled between the output and the input of the operational amplifier. The input resistor includes a primary input resistor and a plurality of input adjustment resistors connected in series with the primary input resistor. The feedback resistor includes a primary feedback resistor and a plurality of feedback adjustment resistors connected in series with the primary feedback resistor. The switching network includes a plurality switches, each of which has a signal input and a signal output. The signal outputs of a subset of the plurality of switches are connected in common with the input of the operational amplifier. Each of the inputs of the subgroup of the plurality of switches is connected to one of either the input adjustment resistors or the feedback adjustment resistors. With this configuration, the value of both the input and feedback resistors may be varied/defined by selectively connecting the adjustment resistors to the operational amplifier input.
An additional embodiment of the operational amplifier includes two inputs and two outputs. A first resistor network is provided that is connected to one of the outputs, with a first switching network connected between the first resistor network and one of the inputs. The remaining output is connected to a second resistor network, with a second switching network connected between the second resistor network and the remaining input. Each of the first and second resistor networks includes an input resistor and a feedback resistor, as discussed above. The first and second switching networks each includes a plurality of switches connected to vary the value of the input resistor and the feedback resistors, as discussed above.
The primary resistors have a value, R, associated therewith, and the adjustment resistors having a combined value, r, associated therewith. Typically, the value of r is in the range of R/100 to R/1000, inclusive. The actual value of the primary resistors are dependent upon the desired gain of the amplifier. In one example the output of the amplifier is ⅓ of the input, providing a gain of ⅓. To minimize gain drift of the operational over, the ratio of feedback adjustment resistors to input adjustment resistors is directly proportional to the differential gain AD.
In operation, the resistance of the input and feedback resistors is dependent upon the operational characteristics desired. In the present example, the important characteristics are the common-mode gain, ACM, and differential gain, AD. Usually, the common-mode gain is established by selectively placing in electrical communication with an input of the operational amplifier, one or more of the adjustment resistors.